Week 10 - Soil chemical environment - soil pH (soil reaction)

Question 1

What is the ideal pH for most plants?

Answer 1

For most plants a pH of between 6.0 and 6.5 is ideal.

Question 2

a) Briefly describe why the accumulation of organic matter/organic acids may change soil pH?

b) What direction will pH change?

Answer 2

a) Dissociation of organic acids release H+.

R - COOH ⟺ R - COO- + H+

b) pH will become more acidic (reduce).

Question 3

Describe two biological processes responsible for causing acidity in soil.

Answer 3

i. Oxidation of NH4+ (the process is called nitrification). The organisms which are responsible for NH4+ oxidation are Nitrosomonas, Nitrobacter spp. They are chemoautotrophs, that is, they obtain their energy from the process.

NH4+ + 2O2 → NO3- + 2H+ + H2O

ii. oxidation of elemental sulphur (the organisms involved are Thiobacillus spp.)
2S + 3O2 + 2H2O → 4H+ + 2SO42-

Question 4

a) Name the various chemical and biological processes adding or removing H+ form the soil solution.

b) When will these processes result in a pH change?

Answer 4

a)

i. Accumulation of organic matter/organic acids
ii. Various biological processes
iii. Removal or return of plant products
iv. Accumulation of CO32-/HCO3- salts
v. Effects of water-logging (reduced aeration)
vi. Soluble Al
vii. Dissolved CO2

b) pH change only occurs when the net of all these processes results in net H+ addition or consumption.

Question 5

How does the removal or return of plant products affect acidity?

Answer 5

Plants normally take up of large concentrations of K+, Ca2+, Mg2+ relative to H2PO4- and SO42-.

This imbalance in the uptake of cations and anions results in plants excreting H+ into the soil to maintain electrical neutrality and their accumulation of organic anions.

Harvest (removal) of plant materials such as hay, grain, grapes, or grass, leaves the H+ in the soil (causes acidification).

Question 6

How does the removal or return of plant products affect alkalinity?

Answer 6

Where organic anions in plant materials such as hay or lawn clippings, are returned to the soil their oxidation releases alkali.

Question 7

How does the imbalance in the concentration of cations and anions taken up by plants depend on the form of N taken up by the plants?

Answer 7

Where NH4+ is the N source for growth, H+ is excreted.

Where NO3- is the N source for growth, OH- (or HCO3-) is excreted.

Where N is derived from N2­ via N fixation, legumes also excrete H+.

Question 8

How does water-logging (reduced aeration) affect pH?

Answer 8

In acid soils, the pH rises following waterlogging because the microorganisms reduce Fe3+ , Mn4+ and NO3- eg.

Fe(OH)3 + 3H+ + e- → Fe2+ + 3H2O

MnO2 + 4H+ + 2 e- → Mn2+ + 2H2O

2NO3- + 12H+ + 10e- -> N2 + 6H2O

In alkaline soils, the pH drops so that the waterlogged soils develop a pH around 7.

Question 9

What can happen if a soil experiences periodic water-logging and drainage over a period of time?

Answer 9

Periodic water-logging and drainage over a period of time can result in very severe pH decline, that is, serious soil acidity.

Question 10

How does dissolved CO2 impact soil pH?

Answer 10

This produces a weak acid H2CO3 which can enter the soil as rainwater or be produced when CO2 from root respiration dissolves in soil water.

Question 11

Explain in detail why is it easier to modify the pH of a sandy soil which is low in organic matter than a heavy clay soil?

Answer 11

If the H+ is removed from the soil solution for some reason, then H+ ions can be released back into the soil solution, for example from:

• H+ on cation exchange sites on clay minerals;
• -COOH groups on organic matter;
• hydrolysis of Al(H2O)63+.

As a result, the pH of the soil solution does not change easily, i.e. the soil pH is buffered.

The extent of buffering ( the pHBC), in soil will depend on factors like;

layer silicate clay content;
sesquioxide content;
organic matter content.

Therefore, pH will be poorly buffered (low pHBC) in sandy soils but will be strongly buffered (high pHBC) in clay or organic soils.

Question 12

What are the reasons plants grow poorly in acid or alkaline soils?

Answer 12

i. H+ toxicity
ii. Supply of Ca2+, Mg2+ and K+ from soil
iii. P availability
iv. Availability of micronutrient cations (Fe2+, Mn2+, Cu2+, Zn2+)
v. Mo availability
vi. Al3+ toxicity
vii. Biological aspects of low pH
viii. Water supply by soils

Question 13

How does pH affect the supply of Ca2+, Mg2+ and K+ from soil?

Answer 13

These are the plant available forms and are normally stored on the cation exchange sites.

However when H+ or Al3+ levels are high as in acid soils, these ions displace the Ca2+, Mg2+ and K+ by cation exchanges which will allow the latter to be leached from the soil.

Problems of deficiency with K+ and Mg2+ in acid soils are the most common as Ca2+ is normally present in higher concentrations.

Question 14

How does pH affect P availability in soil?

Answer 14

Available P levels in soil are normally low in acid soils and alkaline soils.

Availability (solubility) is greatest in the neutral to slightly alkaline pH range.

Question 15

What happens to the availability of micronutrient cations (Fe2+, Mn2+, Cu2+, Zn2+) in alkaline soils?

Answer 15

These are highly soluble in acid soils but their solubility decreases with pH increase.

Thus for Mn which is present in greatest concentrations in soils, Mn toxicity is a problem in acid soils especially at pH less than 5. Conversely, in alkaline soils, especially over pH 7, these can be so insoluble that deficiencies can occur.

Question 16

Why is Al3+ toxicity a problem for plants in acidic soil?

Answer 16

Al3+ is not an essential element but it is toxic when high concentrations of Al are in solution and are taken up by plants.

Al solubility increases as the pH drops especially below pH.

Soluble Al reduces the availability of P in the soil and impedes the processes involving P within plants.

Question 17

What are the biological aspects of low pH?

Answer 17

• Soil microorganisms prefer neutral conditions so release of N, P and S from organic matter can be reduced when pH is low.
• Low pH reduces the ability of many Rhizobium spp. to survive in the soil so there are less to infect legume roots. In addition the nodulation of the legume may be reduced in acid soil.
• Earthworm populations fall in acid soil.
• Some plant pathogens, for example, organisms causing clubroot in brassicas are more of a problem in acid soils. Soil pH of greater than 5.5 can favour take-all in wheat. Thus if acid soils are limed, take-all may become a problem where crop rotations allowed the organism to persist in the soil.

Question 18

How does pH affect water supply by soils?

Answer 18

Water uptake depends on root development and where subsoils are acidic, plant roots will be less likely to grow. As a result, moisture stored in the subsoil will not be available. This is particularly important in drier areas.

Question 19

Recommend an approach to diagnosing acidity/alkalinity problems in soils?

Answer 19

Where a problem of surface acidity is diagnosed, samples from say 15-30, 30-45 and 45-60 cm should be taken and analysed to assess the extent of subsoil problems.

1. Measure soil pH: This can be done by using:
   * colour indicator solutions
   * pH meters.
2. Measure Al levels in soil: It is common to measure extractable Al3+ concentrations. You will often notice that exchangeable Al3+ is determined during routine soil testing for acidity.

Question 20

What strategies can be adopted if an acidity/alkalinity problem is diagnosed in soil?

Answer 20

• change the pH with a suitable amendment
• apply fertiliser to cure the appropriate deficiency
• appropriately pellet (lime/rock phosphate) inoculated legume seed
• grow cultivars which are tolerant of acidity (usually Al, Mn toxicity) or alkalinity.

Question 21

What is a management strategy for reducing the pH of a soil? What are the pros and cons?

Answer 21

Apply an appropriate acidifying material such as NH4+ fertiliser, FeS or elemental sulphur (S).

Normally very expensive and only economic where a high return horticultural crop is involved.

Question 22

What is a management strategy for increasing the pH of a soil? What are the pros and cons?

Answer 22

Apply a liming material such as:

• Agricultural limestone (CaCO3).
• Slaked lime (CA(OH)2)
• Burnt lime (CaO)

Rapid but normally costly. CaCO3 has low solubility. Slaked lime and burnt lime are more soluble but are more expensive, very fine, and caustic (making handling and spreading difficult).

Question 23

Explain how lime pelleting can help in acidic soils.

Answer 23

This involves covering the inoculated legume seed with a lime coat. This enables the Rhizobium spp. to survive in acid soil while the legume seed is germinating, that is before the roots can be infected and the nodule formed.

Question 24

At what soil pH will the following elements become toxic and/or deficient:

a) Mo
b) Mn2+
c) H+
d) Ca2+
e) Al3+

Answer 24

a) Mo: No evidence that high Mo levels are toxic to plants. Deficient in acidic soils.
b) Mn2+: Mn toxicity is a problem in acid soils especially at pH less than 5. Conversely, in alkaline soils, especially over pH 7, Mn can be so insoluble that deficiencies can occur.
c) H+: Toxic below pH 3.5.
d) Ca2+: Deficient when H+ or Al3+ levels are high as in acid soils because they displace Ca2+ from cation exchange sites allowing it to be leached from soil.
e) Al3+: Toxic below pH 5.

Question 25

As soil pH (0.01 M CaCl2) increases from 5 to 8 describe the changes that occur to the concentration of available phosphorous.

Answer 25

In general, P availability is lower at pH extremes, that is low and high pH.

However, in soils that are initially acidic and have high concentrations of exchangeable and soluble Al, following liming, availability of P decreases in the neutral pH range.

Therefore, as the pH increased the available P would decrease between 5 & 6 pH and then increase towards pH 8.

Question 26

How do clay particles and organic matter buffer against changes in soil pH?

Answer 26

If the H+ is removed from the soil solution for some reason, then H+ ions can be released back into the soil solution, for example from H+ on cation exchange sites on clay minerals;
and -COOH groups on organic matter;

As a result, the pH of the soil solution does not change easily, i.e. the soil pH is buffered.

Question 27

How do additions of lime increase the pH of acid soils?

Answer 27

Lime increases pH by the CO32- consuming H+ ions from the soil solution.

CaCO3 + 2H+ → Ca2+ + H2O + CO2

Question 28

Explain why acid soils become neutral following water-logging.

Answer 28

In acid soils, the pH rises following waterlogging because the microorganisms reduce Fe3+ , Mn4+ and NO3- eg.
Fe(OH)3 + 3H+ + e- → Fe2+ + 3H2O
MnO2 + 4H+ + 2 e- → Mn2+ + 2H2O
2NO3- + 12H+ + 10e- -> N2 + 6H2O

pH rises because H+ is consumed in the redox reaction.

Question 29

c) Explain, including equations, the change in soil pH that occurs after ammonium fertiliser is applied to topsoil

Answer 29

Soil pH is reduced because of the biological process nitrification when NH4+ fertilised is applied to top soil. NH4+ is oxidised producing NO3-, H+ and H2O increasing the acidity of the soil.
Equation: NH4+ + 2O2 → NO3- + 2H+ + H2O